This is weird. Branson doesn't do greenwash advertising so much as headline-earning stunts and initiatives that build the aura of a cool, progressive, green guy.

A cynic would say that his $25m Virgin Earth Challenge prize for a new low-carbon technology is worth the price in free editorial. Equally, Branson's initiative on biofuels for aircraft, while slightly tarnished by the declining green credentials of biofuels, also grabbed headlines for what does not, as yet, amount to very much.

Others, including the investigative journalist Tom Bower, have tried to grapple with the business ethics of the Branson brand. So I will stick with the "how green is Branson?" question.

For a start, there is the little matter of his plans to develop space tourism with Virgin Galactic.

More to the point is Branson's airline, Virgin Atlantic. It's not the biggest in the world. Its website says its carbon dioxide emissions are currently approaching 4.8m tonnes. This is up from 4.2m tonnes five years ago, but still way behind British Airways' 17m tonnes.

Perhaps the most surprising comparison is with Ryanair, Europe's largest low-cost airline. Ryanair has a bad environmental reputation, largely because its boss, Michael O'Leary, is fond of taking crude pot-shots at environmentalists, who he dubs "eco-nutters". No Branson, he. But in a head-to-head you may be surprised who comes out top.

But it turns out that the budget airlines, with their newer fleets and policy of filling up their aircraft at all costs, boast significantly lower emissions. Easyjet weighs in at 97.5g. And Ryanair tops the green list with 96g.

Nothing is quite so simple, of course. Ryanair keeps that figure down by selling any spare seats on its flights at rock bottom prices. It is encouraging cheap-and cheerful weekend-break flights that would not otherwise have happened.

Maybe he should go back to the balloons. That way we at least know he is running on hot air.

• This article was amended on Monday 7 September 2009. We said British Airways claims to have reduced its carbon dioxide for every passenger-kilometre to 111 grams; in fact that figure was for 2005. The company's latest figure, for 2008, is 107.3g. This has been corrected.